This invention relates generally to magnetic storage devices and particularly to servo methods for magnetic tape recording systems.
The constantly increasing operational speeds of digital computers are creating a demand for corresponding increases in the data storage capacities of magnetic tape recording and reproducing systems, while maintaining the special requirements of high speed digital tape systems.
Tape recording and reproducing systems for use as computer data storage devices are required to provide high data transfer rates and to perform a read check on all written data. To satisfy these requirements, conventional tape systems typically employ methods of recording known as linear recording, in which the tracks of data lie parallel to each other and to the edge of the tape, or helical scan recording, in which the tracks of data lie parallel to each other but diagonal to the edge of the tape. The linear recording method offers higher data transfer rates; however, it is desirable to obtain higher data densities while retaining the advantages of this method. Various methods of increasing tape track densities have therefore been pursued.
A method of recording known as azimuth recording has been used in helical scan recording system, and has recently been applied in linear tape systems to increase the track density of these systems. Azimuth recording results in a recorded track pattern in which the magnetization directions of adjacent data tracks lie at different azimuth angles to each other. This method greatly reduces inter-track crosstalk, allowing tracks to be placed closer together. The need for guard band spaces between tracks or wide write heads is thus reduced or eliminated.
Tape track densities are further limited, however, by lateral tape motion, which is the random and unavoidable tendency for a tape to drift in a direction lateral to the direction of tape motion. During a tape write, lateral tape motion causes track directions to deviate from the parallel to the edge of the tape. During a read, lateral tape motion causes misregistration of the read head over the track being read. This misregistration results in read data error. Further error can be introduced by lateral motion of the write head during writing.
Servo tracking techniques have been developed to reduce the effects of tracking error and thus improve the data capacity of tape systems. Known servo techniques vary widely, but most involve methods of dynamically moving the read head to continually re-position it over the written data track. The movement of the read head gap compensates for lateral tape motion during a read. However, lateral tape motion during writing is usually not compensated for with respect to the write head; thus, the distance between tracks is still limited to the magnitude of the lateral tape motion in order to avoid over-writing previously written tracks. Known servo techniques are also costly in that they may require the use of pre-formatted tapes or additional heads.
One method of compensating for lateral tape motion during writing involves embedding low frequency servo bursts within the tracks and then using the servo bursts in an adjacent previously written track to control the lateral position of writing a current track. Writing continues during the reading of a first servo burst, and is discontinued during the reading of a second servo burst. The difference in amplitude received from the servo burst reads indicates the proximity of the write head to the previously written track, and the position of writing can be adjusted in response.
This method, however, requires that the read head be capable of discerning servo information in adjacent tracks. The low frequency nature of the servo bursts renders them discernible, but requires the use of a second read channel in addition to the data read channel. Furthermore, relatively complex hardware must be employed to interrupt writing over some servo bursts while continuing writing over others, and to store the information being read for a later comparison.
It is desirable to provide a simplified, low cost servo tracking mechanism to control the effects of tracking error during writing in order to further increase track density. It is then possible to provide a tape system suited for computer data applications in which the data capacities are greatly increased over those of the prior art.